The attachment path is set to .../yabb/attachments, but after you add an attachment and go back into the forum and click on the attached file, it tries to go to ...yabb/YaBB.pl?action=downloadfile;file=photo_1_.JPG. Maybe this isn't the problem. However, I can't seem to find anything else that might be causing this problem. I have tried everything I can think of, which is obviously not enough. (resize pictures, change attachment settings, etc...). Nothing seems to work. Please help.

The attachment path is set to .../yabb/attachments, but after you add an attachment and go back into the forum and click on the attached file, it tries to go to ...yabb/YaBB.pl?action=downloadfile;file=photo_1_.JPG. Maybe this isn't the problem. However, I can't seem to find anything else that might be causing this problem. I have tried everything I can think of, which is obviously not enough. (resize pictures, change attachment settings, etc...). Nothing seems to work. Please help.

Like Captain John says, you must use Absoute Paths and not relative paths..../yabb/attachments - is a realtive Directory Path

Its cPanel, right? So you are looking for the Absolute Path, which should be (depending on you cPanel account)/public_html/yabb/attachments - (or something simillar.)